Enhanced the thermal stability and crystallinity of polylactic acid (PLA) by incorporated reactive PS-b-PMMA-b-PGMA and PS-b-PGMA block copolymers as chain extenders

The reactive block copolymers were introduced as chain extenders to eliminate thermo-hydrolysis, transesterification or depolymerization of polylactic acid (PLA) during the thermal processing. A novel modified living anionic polymerization was employed to conquer the tough conditions of traditional anionic polymerization to synthesis the reactive block copolymers. In the first step, polystyrene capped with thiol end-group which can act as initiators for mercaptan/ε-caprolactam living polymerization was synthesized by anionic polymerization at room temperature. Then, the reactive block copolymers poly(styrene-b-methyl methacrylate-b-glycidyl methacrylate) (PS-b-PMMA-b-PGMA, PSMG) and poly(styrene-b-glycidyl methacrylate) (PS-b-PGMA, PSG), were synthesized via the mercaptan/ε-caprolactam living polymerization as proposed by our previous studied. FT-IR, ¹H NMR, DSC, and POM analyses were used to confirm the structure, characteristics and thermal behavior of the PSMG, PSG and the processed PLAs. Analysis of the processed PLAs showed that their crystallization behaviors were strongly influenced by the segment content of the chain extenders, PSMG and PSG. The GMA segment contains epoxide moieties, which can increase the molecular weight and melt strength of PLA, while the MMA and styrene segments enhance miscibility with PLA and act as nucleating agents to promote crystallization, respectively. Compared to PSMG, PSG did not show a good reactivity and nucleation ability in PLA melt-blending because of lacking MMA segments. This result reveals the MMA segments are the key structure of the chain extender and the optimum ratio of styrene to MMA is at 1.0-1.2. The crystallinity of PLA could increase from 12.29% to 47.54% when these chain extenders, PSMG, were added in PLA melt-blending. By controlling the structure and segment length of PSMG via a novel modified living anionic polymerization route, we can obtain a quickly crystallinization and good mechanical properties of PLA resins.